As is well known in the field of audio equipment, digital record reproducing systems have been generally adopted for the purpose of reproducing recorded data of high density and high fidelity. In these systems, audio analog signals are converted into digitized data by a PCM(Pulse Code Modulation) technique and this data is recorded on a recording medium, such as a disc or a magnetic tape, to be reproduced. Among these systems, the compact disc is mainly used at present. In this case, a binary digit(bit) corresponding to the digitized data is formed on a disc of about 12 cm in diameter, and is optically read.
A reproducing device for reproducing the data recorded on the compact disc as described above, moves an optical pickup provided with a semiconductor laser and a photo-electric conversion device in a constant linear velocity tracking system(CLV) from the inner periphery side to the outer periphery side of the disc, and by rotating the compact disc the data recorded on the compact disc may be read.
The data read out from the disc is supplied to an RF circuit. This RF circuit extracts a focus error signal and tracking error signal from an output of the optical pickup, and supplies a servo control circuit with these extracted signals. The RF circuit also quantizes the signal read from the disc, and supplies a signal processing circuit with the quantized signal as an eight to fourteen bit modulated(EFM) signal. This signal processing circuit operates an EFM demodulation, a subcode demodulation and an error correction processing, then its output is supplied to a D/A(digital/analog) converter(hereinafter called DAC). An output of the DAC is supplied to a low pass filter(LPF), and an output of the LPF corresponds to a reproduced audio output signal. In a reproduction device used with a CD, in order to prevent a final audio output from causing wow and flutter, the EFM demodulated signal is stored in a buffer memory and read out after the error correction processing using a stable frequency clock signal. This clock signal is synchronous with a, clock signal generated by a crystal oscillator.
Besides the well-known audio CD player, a CD-ROM player is another reproducing apparatus using discs. The CD-ROM player reproduces two kinds of data co-existing on the disc, i.e. an audio signal and ROM data, such as image information and character codes. When the audio signal is read out, the reproduction is performed at a normal rate corresponding to audible information. On the other hand, when ROM data is read out, the reproduction is performed at a faster rate, such as a double rate. In such a CD-ROM player, frequent switching from the normal rate to the double rate or from the double rate to the normal rate is performed.
As shown in FIG. 1, to change the reproduction rate, a rate control signal(HS) is used. When the signal HS is changed from "L" to "H", the rate is switched from the normal rate to the double rate, and when the signal HS is changed from "H" to "L". the rate is switched from the double rate to the normal rate. In such cases, the rotation speed of a disc motor does not change instantaneously but changes gradually from the normal rate to the double rate or from the double rate to the normal rate. As a result, a frequency of a reproduction stage clock(VCOCK) generated by a PLL circuit in accordance with the rotation speed of the disc motor flutters frequently in accordance with the motor rotation. The reproduction stage clock controls the rate at which data is written into the buffer memory.
On the other hand, a signal processing stage clock(XCK) for error correction processing and audio output processing is switched promptly. The signal processing stage clock controls the rate at which data is read form the buffer memory. As a result, within a transitional period of the disc motor until the rotation speed reaches the required speed, the frequency difference between the reproduction stage clock(VCOCK) and the signal processing stage clock(XCK) is relatively large.
In the buffer memory, storing(writing) is performed by a reproduction stage frame clock(PFS) synchronous with the clock VCOCK, and reading is performed by a signal processing stage frame clock(XFS) synchronous with the clock XCK which is synchronous with a clock signal generated by a crystal oscillator. For this reason, the address difference between storing and reading caused by the frequency difference exceeds an allowed value for the buffer memory. This means an overflow or underflow, i.e., emptying, which results in a reproduction interruption.
The problem mentioned above is not limited to the CD-ROM player. For example, the problem also exists for a CD player which comprises a shock proof memory to eliminate effects of data pick-up interruption caused by a shock from outside. The shock proof memory, usually a large capacity memory such as 4MB DRAM(four megabit dynamic random access memory), is employed between the buffer memory and the DAC to store the reproduced output of the buffer memory. The reproduced output is stored to the shock proof memory, then read out. This system is called shock proof.
In the shock proof system, storing to the shock proof memory is controlled to keep a predetermined data quantity in the shock proof memory in the following manner. When storing is interrupted by a shock from outside, the reproduced data in the shock proof memory is reduced because reading-out continues. If the quantity reduction in the shock proof memory is detected, then in order to recover the predetermined quantity in the shock proof memory, a reading rate from the buffer memory is switched to a faster rate. In this case, because the rotation speed of the disc motor changes gradually, the difference between the reproduction stage clock and the signal processing stage clock is increased. As a result, an underflow occurs in the buffer memory, which causes a reproduction interruption.
As described above, in systems such as the shock proof system and CD-ROM in which the reproduction rate is switched, the reproduction can be interrupted because of overflow or underflow.
Sudden speed change also occurs, by a search operation, in a conventional CD player which does not employ the reproduction rate switching. The disc rotates in about 8Hz when the pickup is at the inner periphery side of the disc, and 3Hz when the pickup is at the outer, periphery side so as to maintain a constant linear velocity(CLV). For this reason, to track the data in the outer periphery side when the pickup is tracking the inner periphery side, the rotation speed of the disc motor should be reduced to below one-half. In the opposite case, when the pickup is at the outer periphery side, the speed should be increased to two times greater to track the data in the inner periphery side. The frequencies of the clocks VCOCK and PFS obtained by supplying the RF circuit and the PLL circuit with the signal from the pickup, are synchronous with the disc rotation rate. For this reason, at the moment when the pickup moves to a target point, for example from the inner periphery side to the outer periphery side, the frequencies of the clocks VCOCK and PFS are higher than their predetermined value for the target point until the disc motor decreases its speed to the predetermined speed. When the pickup moves from the outer periphery side to the inner periphery side, the frequencies of the clocks VCOCK and PFS are lower than their predetermined value for the target point. These cases can result in underflow or overflow, by the frequency difference between the clocks PFS and XFS, which results in the reproduction interruption. Particularly, in the system in which high-speed access is required, a long muting period until the reproduction is restarted is one of the serious problems.
To solve the above problems, although using high-controllability motors which have a shortened transitional period can be effective, these motors are expensive and consume greater electric power. Use of these motors results in an increased price of the CD player set and decreases the marketability of the CD player set.
In the field of disc data reproduction systems, a disc data reproduction apparatus which uses a clock generated by a PLL (Phase Locked Loop) circuit is suggested. Such a reproduction apparatus is, for example, disclosed in the Japanese Patent Laid Open(Ko-kai) 93-28632. In the reproduction apparatus (player) disclosed in the above laid-open patent, a disc is rotated at a constant angular velocity (so-called, CAV) by a spindle motor. The CAV rotation of the disc means that the linear velocity of the tracking changes according to the rotation radius and data transfer rates are not constant. For example, under constant angular velocity, when the data transfer rate represents four times the normal rate at the outer periphery side, the data transfer rate represents 1.6 times the normal rate at the inner periphery side.
To implement the above mentioned CAV player, a calculation circuit which calculates a reference frequency for the PLL circuit and a frequency synthesizer which controls the frequency are required. The calculation is executed according to the address information and optical pickup position information detected by a position sensor. Further, for the usual audio use such as music reproduction, a CAV player must employ other circuits to perform CLV reproduction. As a result, disc data reproduction using a CAV system has a complex structure and increases the price of the player set.